1. Field of Invention
This invention relates to a control system for a variable pitch horizontal axis wind energy conversion system which utilizes wind energy to produce electrical power, and specifically to a closed loop control system which automatically modulates the pitch angle of the wind turbine blades to provide safe operation and optimize the energy capture from the wind. The wind turbine rotor drives a synchronous generator which is connected to supply electrical power to a power utility grid, the control system being adapted to rapidly modulate the blade pitch angle to minimize the effects of wind gusts and turbulence to maintain desired electrical frequency and phase, and to reduce stress on the rotor blades and other mechanical components.
2. Description of the Prior Art
The recent energy crisis and the high cost of fossil fuels has resulted in renewed interest in and development of large scale wind turbines, in effect large windmills, which rotate and produce energy in response to natural wind currents. The rotor blades of the wind turbines are up to 300 feet in diameter, and the output power when converted into electrical energy is measured in megawatts. While in some wind turbines the output power is used directly to power mechanical devices, in most installations the rotary energy is converted into electrical power which may be stored, used to power isolated installations directly, or fed to power utility grids for ultimate distribution.
A major problem with wind turbines is that of controlling the electrical output phase and frequency in the presence of unpredictable wind gusts and turbulence, particularly when the wind turbine is driving a synchronous generator connected to a large electrical network. At high wind velocities, even mild wind turbulence conditions can create shaft torque fluctuations of sufficient magnitude to cause the synchronous generator to pull out of phase synchronization and be disconnected from the grid.
In an attempt to overcome this problem, variable pitch angle rotor blades have been provided for use in wind turbines with a rapid acting control system to position the blade angle in response to selected operating parameters such as wind velocity, rotor speed and output power. A representative control system of this type is disclosed and claimed in copending application Ser. Nos. 916,320, 916,321 and 916,329, each filed on June 15, 1978 and assigned to the same assignee as the present application, and to which reference may be made for a complete explanation of such control systems. Briefly, the prior art control systems, referenced supra, control the rotor blade pitch angle in a closed loop manner to maintain constant speed or power depending on whether the generator is off-line or on-line, and in an open loop manner during start-up and shutdown.
The present invention is an improvement of the control system referenced supra, and discloses a wind turbine rotor blade pitch angle control which provides improved dynamic performance and transient response characteristics. The improved operation is obtained by closed loop control of rotor acceleration rate and deceleration rate during start-up and shutdown respectively; by use of a single control integrator for all closed loop operating modes, with a rate limiter in front of the integrator to prevent integrator overtravel, the integrator having maximum and minimum blade angle stops, the minimum stop being variable as a function of rotor speed and wind speed; and by a closed loop shaft torque control for on-line power control, the torque control having proportional, integral and derivative control paths in which rate of change of shaft torque is synthesized from the difference between rotor speed and generator speed.
It is therefore an object of the present invention to provide an improved pitch angle control for a wind turbine which modulates the wind turbine blade angle in response to a number of selected operating conditions.
Another object of this invention is an electronic pitch angle control for wind turbines which schedules blade angle is a closed loop manner to minimize blade stress and shaft torque variations during start-up and shutdown transients.
A further object of this invention is a closed loop electronic pitch angle control for wind turbines which regulates speed when the synchronous generator is off-line and regulates torque when the synchronous generator is on-line.
Another object of this invention is the use of a single control integrator for all closed loop control modes of operation.
A further object of this invention is the use of a rate limiter circuit in front of the control integrator to prevent integrator overtravel.
Another object of this invention is the use of a variable minimum blade angle stop on the electronic control integrator scheduled as a function of rotor speed and wind speed.
A further object of this invention is the use of proportional, integral and derivative compensation for on-line torque control, and proportional plus integral compensation for off-line speed control.